Laser liftoff techique to yield biodetection chip

HANCOCK, N.H.  A highly diverse integration technique based on a process called laser liftoff is being combined with system-on-chip processing technology and microelectromechanical-system concepts in a bid to reduce lab-sized laser detection systems to a single chip.

In the past few years, research groups at the University of California-Berkeley and Purdue University have perfected the liftoff technique to produce freestanding LED membranes, blue LEDs on silicon chips, continuous-wave blue-laser diodes on copper substrates and amorphous silicon thin-film transistors that can be transferred to virtually any substrate.

Now researchers are ready to take on sophisticated biodetection applications, reducing lab-sized laser detection systems to a single chip.

Biological applications are being developed in a collaborative effort between the Purdue center and a group led by Luke Lee at UC Berkeley. Sands, Cheung and Lee will announce the biodetection lab-on-chip in a paper that will be published next week in the journal Sensors and Actuators.

The technique was invented at Berkeley in the mid-1990s by Nathan Cheung and William Wong. Cheung subsequently moved to Purdue's Department of Electrical Engineering, while Wong is currently working at the Xerox Palo Alto Research Center. Timothy Sands of Purdue's Heterogeneous Integration Group has worked to perfect the technique in recent years.

Fluorescent detection normally requires benchtop lasers and glass or grating filters that are part of a suitcase-sized detection system. Biomolecules are tagged with a chromophore or metal nanocluster that emits light when irradiated by the laser. The laser light must then be filtered out to detect the signal. Most detection systems require two lasers and two filters. One laser-and-filter system is used to calibrate the signal; the second color is employed for detection.

The ability to integrate more than one optical diode and filter combination on the same chip is said to be a first. Sands expects that the process can be scaled up to produce arrays of LEDs and filters.

The process achieves its high integration flexibility from bandgap-specific liftoff, which offers a precise surgical technique for separating a device from its substrate. For example, gallium-nitride laser diodes or LEDs can be easily grown on sapphire substrates. After the growth process is completed, an excimer laser beam tuned to the bandgap at the interface between the sapphire and the gallium nitride is projected through the sapphire. The effect is to deliver heat only to the first gallium-nitride atomic layer, at the interface with the sapphire. The heat cleanly separates the materials at their interface.

Wong and Cheung combined two techniques into a versatile "pixel-to-point" transfer process that works with a range of materials and devices. Sands and Luke Lee, who heads UC Berkeley's bio-polymer-optical-electromechanical systems (BioPoems) group, began research into biodetection applications. The BioPoems effort is attempting to create an organic-MEMS technology that will be compatible with living cells and other biological components.

The Purdue Heterogeneous Integration group is pursuing other nonorganic applications for laser liftoff. One is attempting to scale the integration of laser diode and LED arrays to wafer-scale systems. One recent success was the transfer of a 2-inch wafer of LEDs to a silicon substrate.